1. Field of Art
This invention relates to tapped, surface acoustic wave (SAW) signal processors, and more particularly to processors employing phase-inverted tap pairs for differential operation that provides significant unwanted component cancellation.
2. Description of the Prior Art
Surface acoustic wave (SAW) signal processing is well known, and may be employed to perform a variety of signal combining/comparing functions, some of which are described in Reeder and Gilden U.S. Pat. No. 4,016,514. These include correlation, convolution, time inversion, and the like. When the SAW signal processors include programmability of the taps, to provide a phase and amplitude programmable, general transversal filter, as in Reeder and Grudkowski U.S. Pat. No. 4,024,480, additional functions, such as programmable correlation, multiplexing, and the like may be performed. Programmable taps may be used in conjunction with other SAW device parameters to provide still additional functions, such as discrete Fourier transformation in a parallel channel processor, as disclosed in Reeder U.S. Pat. No. 4,114,116.
The processors described hereinbefore employ nonlinear product mixing which is achieved in external devices. As is well known in the art, the output of nonlinear product mixing, such as may be effected in suitably biased diodes, includes many components, of which the two fundamental frequencies are the most dominant because they are the strongest and also because they have the largest transfer characteristic within the mixing process. Because the signal processing itself, such as correlation or convolution, manifests its results in those components of product mixing which are generated by the combination of the two fundamental frequencies, and are jointly dependent on both frequencies, it is common to select either the sum or difference frequency as the desired output and use a band-pass filter to significantly reject the remaining components of product mixing, as well as the fundamental components. Summation of the outputs at the sum or difference frequency of many identical taps of a P tap array provides a signal representation of the desired convolution or correlation summation between the two input waves, or between one or both waves and any tap weighting which may be involved. Although the correlation function is not itself hampered by variations in amplitude of the components of incoming signals being correlated, when amplitude weighting is employed, the resultant component at the output of the mixer for any one tap can be much lower than the signal strength of components of product mixing for others of the taps. The filtering out of the desired components to extract an extremely weak signal from amongst much stronger signals therefore becomes difficult. Additionally, unless perfect isolation is provided, which is generally not feasible, unwanted mixing of components from one tap within the mixer of another tap can occur. As disparity in amplitude weighting is increased, the likelihood of desired mixer output components being near the same level as undesired mixer output components is also increased. Resulting spurious signals and difficulties of filter design combine to provide amplitude and phase errors which become progressively worse with increased diversity in amplitude weighting.
An example of the need for sophisticated output filtering is given in Colvin, R. D. et al, Monolithic SAW Mixer with Single Sideband Output, IEEE 1978 Ultrasonics Symposium Proceedings. Therein, dual channel mixing having stagger-tuned output transducers for wider bandwidth is described; it, however, requires a dual channel SAW filter for sufficient spurious signal suppression.
Phase reversal of piezoelectric waves detected by segmented taps, by selection of different segments for the common and distinct outputs with respect to the tap phase center, is well known as illustrated by Hunsinger, B. J., et al, Programmable Surface-Wave Tapped Delay Line, IEEE Transactions on Sonics and Ultrasonics, Vol. SU-18, No. 3, July 1971, pp 152-154; and by Moore et al U.S. Pat. No. 3,942,135. Also, phase reversal of the results of internal product mixing is known in Davis, K. L., Zinc Oxide-On-Silicon Programmable Tapped Correlator, IEEE Ultrasonic Symposium Proceedings, pp 456-458. However, in Davis, all generated signals are phase reversed when the tap phase is reversed.
In addition to the aforementioned Reeder patent, multichannel SAW devices for various purposes are disclosed in Speiser et al U.S. Pat. No. 3,723,916; in Quate U.S. Pat. No. 3,803,395; and in Grudkowski, T. W., and Reeder, T. M., Programmable Transversal Filtering Using Nonlinear Tapped Delay Lines, IEEE 1977 Ultrasonics Symposium Proceedings, pp 710-714.